The referenced Hirt et al Patent 4,833,990 describes a printing form image carrier within a printing press which is coated with ferroelectric material. An electrode pair and a heat source are provided for localized polarization or depolarization, respectively, the electrodes being controlled by an information transmitting unit. The system uses the characteristic of ferroelectric material that differently polarized locations of the ferroelectric material have respectively different affinity for ink and water. Polarizing the printing form in accordance with an image to be reproduced is obtained by spontaneous flip-over of selected regions, which are actually domains, within the material, under the influence of an electric field. It is typical for ferroelectric materials that this spontaneous polarization occurs when a predetermined field strength, depending on the material, is provided, the field strength being referred to as the coercitive field strength of the material.
Once the material, or a region thereof, has been polarized, it remains in the previously generated polarized state. This state is stable, and will be obtained by building an electrical field within the interior of the material due to the charge applied to the surface. The electrical field within the material aligns the ferroelectric domains upon polarization. They will form, fixed in location or space, a double layer of charge and counter charge formed by a dipole. This alignment can be destroyed only by strong external fields or by high temperature; in other words polarizing the material can be changed to depolarization or reverse polarization only by an electric field of the same strength, but in opposite direction or, respectively, by heating above the Curie temperature level, or Curie point. Only when the required charge quantity necessary for spontaneous polarization can flow to the surface of the printing form, polarization can be obtained; this means that the product of current x time must have a predetermined and suitable high level.
In accordance with the Hirt et al patent, pin or strip electrodes can be used. Charge transferred to the surface of the ferroelectric material is obtained by contact or micro discharge in a gap between pin electrodes and the surface of the printing form. An abrasive loading is applied to the surface, and the charge which is transferred may not always be sufficient.